1. Field of the Invention
The present invention relates to a frequency stabilizer, and more particularly to a frequency stabilizer for use in a time-division multiplexed digital cellular system.
2. Description of the Relevant Art
One known digital cellular system is a time-division multiplex access (TDMA) digital cellular system. One conventional frequency stabilizer will be described below with reference to FIG. 1 of the accompanying drawings.
As shown in FIG. 1, the conventional frequency stabilizer comprises a phase error measuring circuit 301 for outputting phase error information in response to a received signal applied thereto, a phase error integrating circuit 302 for integrating the phase error information supplied from the phase error measuring circuit 301, a phase-to-frequency converter 303 for converting the integrated phase error information outputted from the phase error integrating circuit 302 to frequency error information, and a frequency control circuit 309 for controlling the frequency of the receiver with the frequency error information outputted from the phase-to-frequency converter 303.
The conventional frequency stabilizer operates as follows: A radio signal which has been modulated by .pi./4DQPSK (.pi./4 differential quadrature phase-shift keying) and received by a receiver is applied at all times to the phase error measuring circuit 301, which converts the applied signal to a phase error signal. The phase error signal is integrated by the phase error integrating circuit 302. If the receiver has a frequency error with respect to the received signal, then the phase error integrating circuit 302 outputs phase error information. The phase error information outputted from the phase error integrating circuit 302 is converted by the phase-to-frequency converter 303 to frequency error information, which is then outputted to the frequency control circuit 309. The frequency control circuit 309 controls the frequency of the receiver based on the supplied frequency error information.
The frequency stabilizer operates to equalize the frequency of the receiver with the signal frequency. However, the frequency stabilizer cannot control the frequency with high stability unless the receiver is also capable of receiving signals in other slots than the slot in which the signal is being received. For effecting the frequency control at all times, it is necessary to turn on the power supply of the receiver at all times. Therefore, the frequency stabilizer requires the use of a large amount of electrical energy.